Over and across are both prepositions that are used when someone needs to get to the other side of something, for instance the other side of a bridge or a road.
Here are the rules:
1 Getting Over Something Higher: Over
When getting to the position on the other side means getting over something that is high(er), use over.
Example:
They climbed over the wall.
2 Reaching a position over something flat: Across
Use across when the position on the other side is reached on something flat, for instance when it’s a floor or the sea. Across is also used for a country.
Examples:
The dancers moved across the ballroom.
We drove across the United States.
3 Rule 1 and 2 Don’t Apply: You can use both.
When rule 1 and 2 don’t apply, you can use both “over” and “across.” In that case, the position on the other side isn’t higher, or reached over something flat, and you can choose what to use.
Example:
The car drove across the bridge. (okay)
The car drove over the bridge. (okay too)
Their eyes met across the room…
Exercise:
Choose between “over” and “across.” In some cases, both are possible.
You can do this exercise online here.
1 Their eyes met ___________ the room.
2 What do you say, do you want to go hiking straight ___________ those fields?
3 The hikers planned to hike right ___________ the mountain top.
4 His blind fingers moved ___________ her face.
5 Illegal immigrants were trying to get __________ the border.
6 Strangely enough, the escaping kangaroo didn’t jump ___________ the fence, it crawled under it.
7 The ball rolled ___________ the beach.
8 The balloon disappeared ___________ the hill.
9 The train rode ___________ India.
10 The school is just ____________ the road from us.
11 Sergio was too small to look ____________ the window sill.
12 By the end of the flight, empty cups and packages were scattered ___________ the whole plane.
Want to do more exercises with prepositions?
Have a look here:
Prepositions with Water and Sea
Using correct prepositions when you speak your opinion
Afraid of or afraid from and other common mistakes
Which preposition comes after care?
English with a Smile 